According to Ariel, Ferdinand's father is 30 feet under the sea.
Explanation:
Ariel's song features in the Tempest's Act i, scene II. The very first line mentions the information sought here.
It says: Full fathom five thy father lies. One fathom is equivalent to 6 feet so five fathoms will be about 30 feet. Full fathom five... is part of the second stanza of Ariel's song, which is more popular than the first stanza and is often presented in a standalone manner.
This stanza is believed to the source of catchphrases made of identical words and the phrase 'sea change'.
